In your case, there's no need to iterate through the list, because you know which object to delete. You have several options. First you can remove the object by index (so if you know, that the object is the second list element):

a.remove(1); // indexes are zero-based

Then, you can remove the first occurence of your string:

a.remove("acbd"); // removes the first String object that is equal to the
// String represented by this literal

Or, remove all strings with a certain value:

while(a.remove("acbd")) {}

It's a bit more complicated, if you have more complex objects in your collection and want to remove instances, that have a certain property. So that you can't remove them by using remove with an object that is equal to the one you want to delete.

In those case, I usually use a second list to collect all instances that I want to delete and remove them in a second pass: